1. Field of the Invention
This invention relates to an image reproducing apparatus which subjects image information to signal processing of a predetermined system to obtain image signals, then forms a latent image on an image retainer on the basis of the image signals and obtains an image composed of dots by depositing a toner on the latent image in an electric field. More particularly, the present invention relates to an image reproducing apparatus of the type described above which can reproduce an image under conditions suitable for the kind of images to be reproduced.
The present invention relates also to an image reproducing apparatus which develops a latent image formed by different latent image formation means on an image retainer, using developing means in an electric field.
2. Description of the Prior Art
An electrophotographic reproducing apparatus using a laser as an exposure light source such as shown in FIG. 1 has been known in the past.
This reproducing apparatus operates in the following manner. A signal applied from an original camera element or other appliance or an image data I obtained from a data memory or the like is processed by a signal processing device 1 to obtain an image signal (hereinafter referred to as a "binary image") constituted by picture element data that has been converted to binary data (that is, black and white data). A laser write-in device 2 consisting of laser, a sound-optical modulator, lenses, a rotary polygon mirror and the like is then subjected to ON-OFF control for each picture element by the picture element data of this binary image, in order to expose the image by a laser spot on the surface of a photosensitive drum 3 which is uniformly charged electrically by a charger 4 and rotates in a direction represented by an arrow. A toner is then deposited on the resulting latent image by a developing device 5 the details of which are shown in FIG. 2, in an electric field, and the resulting toner image is transferred to a recording paper P that is being fed in such a fashion as to come into contact with the surface of the photosensitive drum 3 in synchronism with the revolution of the drum 3, by transfer means 6. The transfer paper P on which the toner image has thus been transferred is separated from the surface of the photosensitive drum 3 by separation means 7, and the toner image is then fixed by a fixing device 8. The recording paper P is discharged from the reproducing apparatus, and the surface of the photosensitive drum 3 after the transfer of the toner image is cleaned by charge eliminating means 9 to remove the charge. The remaining toner is then removed by a cleaning device 10, thereby completing one image reproducing process.
Next, the developing device shown in FIG. 2 will be explained. Reference numeral 51 represents a developing sleeve made of a non-magnetic material such as aluminum, stainless steel or the like. When a bias voltage is applied to the sleeve from a bias power source 11, an electric field is generated in a developing zone A between the sleeve 51 and the photosensitive drum 3 which has its base portion grounded. A magnet 52 having a plurality of N and S poles is disposed inside, and on the surface of, the developing sleeve 51. When the developing sleeve 51 is stationary or rotates leftward and the magnet 52 rotates rightward or is stationary, respectively, the developer that has been attracted from a developer bin 53 to the surface of the developing sleeve 51 by the magnetic force of the magnet 52 moves counter-clockwise when one or both of these members rotate. The quantity of the developer thus conveyed is controlled by a layer thickness limit blade 54, and a developer layer having a uniform thickness is formed. This developer layer develops the latent image on the photosensitive drum 3 in the developing zone A in which the electric field is generated by the bias voltage. The remaining developer layer after leaving the developing zone A is removed from the surface of the developing sleeve 51 by a cleaning blade 55 and is sent back to the developer bin 53. The developer in the developer bin 53 is agitated by agitation blades 56 and is uniformly mixed with the toner that has been supplied from a toner hopper 57 by a toner supply roller 58.
The signal processing system in the signal processing device 1 in the conventional image reproducing apparatus described above is such that each picture element of the image data I having a multilevel or continuous tone is compared with a threshold value that has been set in advance, in order to convert the image data I to the binary image. This processing system can be classified into a method which sets the same threshold value for all the picture elements (hereinafter called the "pure binary method") and a method which has a different threshold value depending upon each picture element (hereinafter called the "dither method"). The pure binary method is effective for reproducing an image such as a line or a character for which high resolution is fundamentally necessary.
However, when an image such as a photograph for which tone is a requisite is converted to a binary image, a false profile manifests itself and smoothness is lost. On the other hand, the dither method can reproduce artificially the intermediate tone by means of the density of spatial distribution of black dots and hence is effective for processing images such as photographs. However, resolution is reduced.
This point will be described with reference to FIG. 3.
FIG. 3 illustrates the principle of binary conversion of the image data I in the signal processing device 1. The drawing shows the case where a systematic dither method is employed as an example of the dither method. Symbol I1 represents input image data representing the density level of the picture element at 16 stages, R1 is a predetermined threshold matrix, and S1 is output image data which compares the picture element of the input image data I1 with the corresponding threshold value of the threshold matrix R1 and produces a logic "0" or logic "1" depending upon whether the density level of the picture element is above or below the threshold value. The tone can be artificially represented by the distribution of "0" and "1" in the output image data S1 with "0" and "1" representing white and black, respectively, but resolution is obviously less. A binary image having high reproducibility of tone can be obtained by other suitable dither methods besides the systematic dither method shown in FIG. 3, such as a system which sets at random the threshold values "1-16" for each picture element or a system which sets the threshold value for a particular picture element from adjacent picture element data. In comparison with these other systems, the systematic dither method has a higher calculation speed and provides higher reproducibility of intermediate tone.
In comparison with the dither method described above, when the input image data I1 is converted to binary values of "0" or "1" with the threshold value being at "9", for example, in accordance with the pure binary method, the arrangement of "10" in the input image data I1 proves as such to be the black picture element of "1", and the tone can not be represented smoothly with that distribution. Conversely, resolution of the input image data I1 can be maintained.
It can not be said generally that the dither method is more advantageously used in the case where the input image data I has virtually been converted to binary values such as a screen image and the intermediate tone has been artificially reproduced, because moire is likely to occur due to the spatial frequency of dots and the spatial frequency of the dither pattern. It is more advantageous from time to time to process a screen image by the pure binary method.
As described above, an excellent reproduced image can be obtained by forming a binary image in accordance with an appropriate signal processing system depending upon the kind of input image data I. For example, the pure binary method is suitable for line images and screen images, while the dither method is suitable for tonal images.
However, the conventional image reproducing apparatus has converted the image data I to a binary image with the same signal processing system regardless of the kind of images.
In the conventional image reproducing apparatus, the laser write-in device 2 exposes the image with a laser spot having the same diameter regardless of the kind of images to be reproduced, and the resulting electrostatic latent image is developed under an electric field generated by the bias voltage under the same conditions unless specifically changed so as to be otherwise. As a result, the reproduced image consists always of dots of the same size.
This point will be described in further detail with reference to FIG. 4.
FIGS. 4(a), 4(b) and 4(c) illustrate the reproduced image in magnification, and schematically depicts the case where the dot diameter is greater with respect to the gaps between dots (represented by circles) in the order of FIGS. 4(a), 4(b) and 4(c). Generally, high resolution and sharpness are required for line images expressing characters and line images, and in order to satisfy this requirement, the gaps between the dots such as shown in FIG. 4(a) and concavo-convexity such as shown in FIG. 4(b) must not be remarkable. In other words, it is necessary for a line image that the dots continue and overlap one another to form an image such as shown in FIG. 4(c). In the case of the tonal image such as a photograph for which the reproducibility of intermediate tone and smoothness are important, the dots must artificially reproduce the intermediate tone, and to accomplish this object, the spatial frequency must be high, that is, the dots must be arranged in such a fashion that they do not cluster at one position, as described already. The tone of the original image can be easily reproduced if the dots are arranged in such a fashion that they do not overlap one another and the number of dots is proportional to the area to be colored. This means that if the original image is a tonal image consisting of a continuous tone or dots, the arrangement such as shown in FIG. 4(b) in which the dots do not overlap one another or the arrangement such as shown in FIG. 4(a) in which the dots are discontinuous is preferably employed.
The conventional image reproducing apparatus neglects this important point, but reproduces the image by dots of a predetermined size, regardless of the kind of images, as described already. Therefore, it has been extremely difficult to obtain both excellent line images and excellent tonal images.
A image reproducing apparatus has also been known to this data which is equipped with a slit exposure device and a laser spot exposure device, and which develops an electrostatic latent image formed by these devices on the surface of a photo-sensitive member of an image retainer using the same developing means to which the same bias voltage is applied. The image reproducing apparatus satisfies the function of a copying machine using a slit exposure device and the function of a printer using a laser spot exposure device, and moreover, it can reproduce a composite image by superposing a toner image by the use of the slit exposure device on the image retainer with a toner image by the use of the laser spot exposure device. If two different color toners are used for the developing means in this case, the image reproducing apparatus can change the color of the image formed by the slit exposure device from the color of the image formed by the laser spot exposure device.
Analog latent image forming means such as a slit exposure device which projects the image of an original as such on the surface of the image retainer forms, in principle, an electrostatic latent image of a continuous tone, hence it is necessary that development of such a latent image can well reproduce a continuous tone. On the other hand, digital latent image forming means such as a laser spot exposure device which is driven by digital electric signals or the like and radiates the spot exposure light to the surface of the image retainer generally forms a latent image of a dot composition having dispersed tone including binary tone, hence it is necessary that development of the latent image can develop the dots in a high density, but reproducibility of the intermediate tone is not generally required. This means that the conditions required for the image quality vary depending upon the kind of tone of the latent images such as a continuous tone or a dispersed tone, and the condition of development or the developing characteristics must change accordingly.
As described above, however, the conventional image reproducing apparatus effects development under the same developing condition even when the kind of latent image is different. When reproduction is made with high tone, development of the latent image formed by digital means involves the problem that the dots become so small that the degree of breakage of lines and steps is unacceptable and image density is inadequate. When a sharp line image is also to be obtained from the latent image formed by digital means by improving the image density, a problem occurs in that development of the latent image that is formed by analog means exhibits low tone and provides a hard impression. Thus, it has by no means been easy to develop latent images under conditions suitable for each kind of latent image.